Process for electrolytically pickling chromium-containing stainless steel

ABSTRACT

In a process for the electrolytic pickling of chromium-containing stainless steel, pickling proceeds initially in an aqueous Na 2  SO 4  solution and subsequently in acid, preferably mixed acid. The CrO 4   2-   formed is converted by the addition of reducing agent and acid as a function of the pH value and the redox potential to Cr 2  (SO 3 ) 3  or Cr 2  (SO 4 ) 3 . H 2  SO 4  serves as the acid and a substance is used as a reducing agent selected from the group Na x  H y  S z  O v , wherein x=0 to 2, y=0 to 2, y=1 to 6 and v=2 to 6. The pH value is adjusted to less than three by the addition of acid.

FIELD AND BACKGROUND

The present invention relates to a process for the eloctrolytic picklingof chromium-containing stainless steel, in which initially picklingtakes place in an aqueous Na₂ SO₄ - solution and thereafter in acid,preferably mixed acid, optionally without electric current. Such aprocess as described in AT-PS No. 252.685, wherein hydro-fluoric acidplus nitric acid serve as the mixed acid. In the first step themillscale is removed; whilst in the second step the chromium-depletedlayer underneath the millscale, formed during annealing, is dissolvedoff. That process has gained worldwide recognition, e.g. for thepickling of stainless steel strip, but suffers from the disadvantage,that the Cr in the millscale is oxidised by the current to CrO₄ ²⁻,whilst Fe, when dissolved, is precipitated immediately as Fe(OH)₃. TheCrO₄ ²⁻ which is formed, remains in solution and is only removed duringde-sludging, respectively together with the stainless steel strip and isonly then detoxified by the reducing agent. For that purpose, primarilythe reduction with an aqueous FeSo₄ -solution in the pH-range of 0-2respectively 7-8 have found acceptance. In both cases it is necessarysubsequently to neutralise again, in order to precipitate all metal ionsin the solution in the form of hydroxide. Moreover, in the event of asudden availability of very large quantities of CrO₄ ²⁻ -containingsolutions, there always exists a risk of a breakthrough of CrO₄ ²⁻,which may then enter into the effluents. A further drawback of theprocess is that only a fraction of the CrO₄ ²⁻ formed is removed fromthe aqueous solution by the Na₂ SO₄, whilst the balance increases theconcentration in the solution and results in increased attack onplastics pipelines and pumps.

In the publication "Chemical Abstracts", Vol. 87, No. 14, Oct. 3rd,1977, on page 396, Abstract No. 108322s, the formation of CrO₄"respectively Cr₂ O₇ " is indeed described, but no solution to thisproblem is offered.

GENERAL DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a process of the typedefined in the introduction which avoids the aforesaid drawbacks.

This object is attained according to the invention in that acid and areducing agent are added to the aqueous Na₂ So₄ - solution in accordancewith the pH-value and the redox potential, such that a pickling solutionis obtained which is free of CrO₄ ²⁻.

A further development of the inventive concept provides that thepH-value of the solution is adjusted to less than 3, preferably to 1.5to 2.5, advantageously 2, by the addition of H₂ SO₄. In a furtherdevelopment of the inventive concept, provision is made that the redoxpotential of the solution as measured in relation to a calomel electrodeis reduced by the addition of acid and reducing agent by 50 to 100 mV, areducing agent being used in the form of a substance selected from thegroup Na_(x) H_(y) S_(z) O_(v), wherein x=0 to 2, y=2 to 2, z=1 to 6 andv=2 to 6, and that in the reaction Na₂ SO₄ is formed as well.

The following reaction mechanisms may illustrate the manner in which thereduction proceeds:

    3 Na.sub.2 So.sub.3 +3H.sub.2 SO.sub.4 +2 H.sub.2 CrO.sub.4 →Cr.sub.2 (SO.sub.4).sub.3 +3Na.sub.2 SO.sub.4 +5 H.sub.2 OReaction 2

    6 Na.sub.2 S.sub.2 O.sub.3 +6H.sub.2 SO.sub.4 +2H.sub.2 CrO.sub.4 →Cr.sub.2 (SO.sub.4).sub.3 +3Na.sub.2 SO.sub.4 +3Na.sub.2 S.sub.4 O.sub.6 +8H.sub.2 O                                       Reaction 2

    3Na.sub.2 S.sub.2 O.sub.5 +3H.sub.2 SO.sub.4 +2H.sub.2 CrO.sub.4 →Cr.sub.2 (SO.sub.4).sub.3 +3Na.sub.2 S.sub.2 O.sub.5 +5H.sub.2 OReaction 3

    3Na.sub.2 S.sub.2 O.sub.5 +3H.sub.2 O ⃡6NaHSO.sub.3

    6 NaHSO.sub.3 +2H.sub.2 CrO.sub.4 →Cr.sub.2 SO.sub.3).sub.3 +3Na.sub.2 SO.sub.4 +5H.sub.2 O

    3 Na.sub.2 S.sub.2 O.sub.4 +3H.sub.2 SO.sub.4 +4H.sub.2 CrO.sub.4 →Cr.sub.2 (SO.sub.4).sub.3 +Cr.sub.2 (SO.sub.3).sub.3 +3Na.sub.2 SO.sub.4 +7 H.sub.2 O

    3 Na.sub.2 S.sub.2 O.sub.5 +2H.sub.2 CrO.sub.4 →Cr.sub.2 (SO.sub.3).sub.3 +3Na.sub.2 SO.sub.4 +2H.sub.2 O          Reaction 4

    3Na.sub.2 S.sub.2 O.sub.6 +3H.sub.2 O ⃡3NaHSO.sub.3 +3NaHSO.sub.4

    3Na.sub.2 S.sub.2 O.sub.6 +2H.sub.2 CrO.sub.4 →Cr.sub.2 (SO).sub.3 +3Na.sub.2 SO.sub.4 +2H.sub.2 O

In addition the dissolved Fe₂ (SO₄) ₃ will also be reduced by thereducing agent, as exemplified in the following:

    Fe.sub.2 (SO.sub.4).sub.3 +Na.sub.2 SO.sub.3 +H.sub.2 O→2FeSo.sub.4 +2NaHSO.sub.4

and the Fe₂ SO₄ thus formed will react further with H₂ CrO₄ according tothe following reaction equation:

    2H.sub.2 CrO.sub.4 +6 FeSO.sub.4 +6H.sub.2 SO.sub.4 →Cr.sub.2 (SO.sub.4).sub.3 +3Fe.sub.2 (SO.sub.4).sub.3 +8H.sub.2 O,

which once again adds up to the overall reaction according to reaction1.

Due to the oxidation of these substances Na₂ SO₄ is formed as well whichin turn serves as a conductor salt in the Na₂ SO₄ solution, and by anappropriate selection of the pH value of this solution it is possible tocause the dissolved Fe³⁺ to precipitate after the solubility has beenexceeded in the form of Fe(OH)₃, the result of which is that thesolution, after a certain Fe concentration has been reached, need not bediscarded completely, but need only be freed of Fe(OH)₃ sludge. Theconcentration of Na₂ SO₄ in the solution amounts to 10 to 250 g/l,preferably 170 to 200 g/l.

The pH value for these reactions is selected according to the inventionlower than 3, preferably 1.5 to 2.5, advantageously 2. At that pH valuethe reaction rate in the solution is adequately high and the redoxpotential measured against a calomel electrode in the CrO₄ ²⁻-containing solution exceeds that of the solution free of CrO₄ ²⁻ by 50to 100 mV. It stands to reason that in addition to the redox potentialother analytical methods may be utilised as well for the determinationof the CrO₄ ²⁻ -content of the solution, although that particular methodwas found to be the simplest and most cost effective.

SPECIFIC EMBODIMENTS OF THE INVENTION

The following illustrative examples should be read against thebackground of and together with the preceding general description toenable the skilled person to practise the invention with the claimedscope thereof.

EXAMPLE 1

A stainless steel strip 1000×6.0 mm, having a strip velocity of 8.4m/min was pickled in an electrolytical pickling plant with an aqueoussolution of Na₂ SO₄ whereafter the chromium depleted layer positionedbelow the millscale layer was removed in a mixed acid trough usingnitric acid-hydrofluoric acid. In a freshly made up aqueous Na₂ SO₄solution the rise of the Cr⁶⁺ concentration amounted to 0.2 g Cr⁶⁺ /1over a period of eight hours.

After adjustment of the pH value to 2.0 by addition of H₂ SO₄, 96%, thetotal Cr⁶⁺ was reduced by the further addition of 8.8 ml 10% Na₂ SO₃solution and 3.7 ml 96% H₂ SO₄ per liter of the aqueous solution whilstthe redox potential of the solution changed from a previous 620 mV to530 mV, measured against a calomel electrode.

During the subsequent eight hours this redox potential was kept constantby further constant addition of Na₂ SO₃ solution as well as sulphuricacid. Towards the end of the eight hour period no further Cr⁶⁺ could bedetected analytically in the aqueous solution.

EXAMPLE 2

Subsequent to Example 1 the addition of reducing agent was stopped untilthe redox potential had again risen to 620 mV. After approximately afurther 4 hours the analytically determined Cr⁶⁺ concentration amountedto 0.11 g Cr⁶⁺ /1. By the addition of solid Na₂ S₂ O₅ in an amount of0.9 g Na₂ S₂ O₅ (62%) per liter, it was possible to reset the redoxpotential once again to 520 mV, as measured against a calomel electrodeand analytically no Cr⁶⁺ could be detected any more. During the additionof the Na₂ S₂ O₅ the pH value of the solution dropped from 2.0 to 1.9.

EXAMPLE 3

After the addition according to Example 2 the addition of a reducingagent to the aqueous solution was interrupted once again until again aCr⁶⁺ concentration of 0.16 g Cr⁶⁺ /1 had been attained. By the additionof 3.9 ml 10% Na₂ S₂ O₄ solution as well as 1.3 ml 96% H₂ SO₄ per literof solution the redox potential was reset to 515 mV and no Cr⁶⁺ could bedetected any more analytically.

In all examples the stainless steel strip was free of millscale and hada silvery lustre after the treatment with acid or mixed acidrespectively.

The claims which follow are part of the present disclosure.

What we claim is:
 1. Process for the electrolytic pickling ofchromium-containing stainless steel, in which initially picklingproceeds in an aqueous Na₂ SO₄ solution and thereafter in acid,comprising the improvement of adding acid and a reducing agent to theaqueous Na₂ SO₄ solution in amounts regulated in accordance with pHvalue and the redox potential of the solution such that a picklingsolution free of CrO₄ ²⁻ is attained.
 2. Process according to claim 1,wherein the pH value of the solution is adjusted to less than 3 by theaddition of H₂ SO₄.
 3. Process according to claim 2, wherein the pH isadjusted to 1.5 to 2.5.
 4. Process according to claim 1, wherein theredox potential of the solution measured against a calomel electrode isreduced by 50 to 100 mV by the addition of acid and the reducing agent.5. Process according to claim 1, wherein a compound of the formulaNa_(x) H_(y) S_(z) O_(v) is used as the reducing agent, wherein x=0 to2, y=0 to 2, z=1 to 6 and v=2 to 6; and wherein in the reaction Na₂ SO₄is formed as well.
 6. Process according to claim 1, wherein the Na₂ SO₄concentration of the solution is adjusted to 100 to 250 g/l.
 7. Processaccording to claim 6, wherein the Na₂ SO₄ concentration is adjusted to170 to 200 g/l.